Treating hydrocarbon oils



March 6, 1934. c. w, MacKAY 1,950,058

TREATING HYDROCARBON OILS Filed May 19, 1950 '0 '0 l 22 o a N V N t 3 l;K) E no Q q Q N h INVENTOB BY A H/ ATTORNEY Patented Mar. 6, 1934 UNITEDSTATES PATENT TREATING HYDROCARBON OILS tion of Delaware Application May19, 1930, Serial No. 453,462

2 Claims.

This invention relates to the cracking of hydrocarbon oils and pertainsespecially to a process wherein the oil is subjected to cracking in aplurality of stages. The invention contemplates in general a process inwhich the oil to be converted or resultant residues of conversion arepassed through a plurality of stages of progressively reduced pressures.

The invention has in View a process in which the oil is passed through aseries of successive cracking and distilling stages in which thepressure is lowered from one stage to another so that, although crackingmay be carried on in the several stages, distillation for separating out1 evolved products may be facilitated by reason of the reductions inpressure. The invention contemplates as a preferred method of operationthe introduction of fresh charging stock to an intermediate stage of thesystem to thus subject :0 the charging stock to cracking at theintermediate pressure and then passing a resultant residue to anothercracking stage or stages operating at lower pressures while refluxcondensate obtained from the system is directed to a higher pressurecracking stage.

The invention contemplates one method of operation in which the oil isdistilled and cracked in a plurality of stages under varying pressureand temperature conditions and the evolved vapors removed from theseveral vaporizing stages and conducted to a common fractionating zonewherein they are fractionated under a pressure preferably lower thanthat obtaining in the several vaporizing stages. The invention has inview the fractionation of the vapors derived from several cracking anddistilling stages to separate out a light gasoline or naphtha distillateand a cycle stock which is adapted to be conducted to a predeterminedcracking and vaporizing stage.

In carrying on the cracking of hydrocarbon oils in a commercial way theusual charging stock is a composite of many hydrocarbons of varyingboiling points. The temperature and pressure requirements best adaptedfor cracking the several component fractions in the charging stock varyand consequently when this composite charge is introduced to a crackingzone it is impossible for the conditions therein to be best suited forall of the several fractions in the stock. This condition may beaggravated in ordinary plant operations by the practice of using as acharging stock a mixture of virgin stock and cycle stock. This cyclestock is obtained as the result of previous cracking operations, beingobtained either directly from a cracking unit or as the result of thedistillation or stripping of oil or residue Withdrawn from a crackingoperation. As stated, it is common practice to utilize as a crackingstock to a cracking unit a mixture of this cycle stock and virgin stockand consequently the pressure and temperature conditions applied to themixed stock must necessarily be a compromise between those conditionsbest adapted for the treatment of either stock separately. My inventionseeks to provide a complete process and apparatus adapted for treatingthe oil in a plurality of stages under various temperature and pressureconditions so that the cycle stocks may be thus treated as they areproduced. In this way a straight virgin stock may well constitute theentire initial charge to the system.

In accordance with my invention charging stock is introduced to apredetermined cracking and distilling stage and oil or residue from thisstage is passed to a succeeding stage operating at lower pressure thanthe first and this operation is continued through any desired number ofstages so that while cracking may be carried on in the several stages astripping or distilling operation is also promoted by reason of theprogressive reductions in pressure. In one method contemplated by theinvention the reduction in pressure may be carried on to such an extentthat no further cracking of any consequence may take place in the latterstage or stages, only a, stripping or distilling action taking place inthese stages. a

In order to more fully explain the invention reference will now be hadto the accompanying drawing which is a flow diagram illustrating aspecific example of the invention. v

In the apparatus thus illustrated a system involving three distillingstages is shown but it is to be understood that the inventioncontemplates any number of cracking and distilling stages that it may bedesired to employ. Each of the several stages shown in the drawingincludes a. heating coil and a pair of stills. The several stages asviewed in the drawing from left to right are intended to operate atprogressively reduced pressures. The first stage includes a. heatingcoil 10A and stills 11A and 12--A, the second stage includes a heatingcoil 10--B and stills 1l--B and l2B and the third stage includes aheating coil 10C and stills 11-0 and 12C. In the arrangement shown theheating coil of each stage is connected to one of the stills as 11--A,11B and 11--C and the stills of each pair are interconnected by vaporequalizing lines 13 and liquid flow lines 14. Although a f LTD singlestill or reaction chamber may be employed in each stage, it isadvantageous to use a plurality of stills, preferably two, introducingthe oil from the heating coil into one of the stills as 11A, 11B and 11Cand withdrawing oil largely from the second stills 12A, l2-B, and 12-C.

The coils 10A, 10B, and 10C are suitably disposed in furnaces so thatthey may be heated to the tem eratures desired. A convenlent method ofapplying heat to the coils is to install all of these coils in a singlefurnace applying the highest temperaturegases to the coil l0-A andprogressively less highly heated gases to the coils 10-43, and 10C. Thestills 11-A, 12-A, l1B, 12-B, etc. may be located outside this furnaceand suitably insulated to retard loss of heat or, if desired, they maybe mounted in suitable furnaces or heating chambers.

The oil to be treated may be drawn from a suitable source, not shown, bya pump 15 and directed through a charging line 16 to an intermediatestage of the system, such as the coil 10-B. The fresh charging stockmay, before being introduced to the coil 10-B, be preheated as by heatexchange with vapors or residual products obtained in the system and mayfurthermore be stripped by distillation of lower boiling constituents,such as components of the gasoline range, before being introduced to thecoil 10-B. The oil is heated to a cracking temperature undersuperatmospheric pressure in the coil 10B and the cracking reaction maybe initiated or carried on to such extent as may be desired in the coilafter which it is discharged into the still 11-13. The oil undergoingconversion passes to the second still 12-B and evolved vapors leave thestills through a vapor line '17 having a pressure reducing valve 18.Liquid oil is withdrawn through a transfer line 19 having a pressurereducing valve 20 and is directed to the heating coil 10C wherein theoil is heated under a pressure lower than that obtaining in the stilll2B to the temperature desired which may be a cracking temperature ormay be merely a temperature sufficient for vaporiaation without materialcracking. The oil is passed from the coil 10-C to the stills 11-C and12-0 and the evolved vapors are removed through a vapor line 21 having apressure reducing valve 22, while liquid oil or residue may be withdrawnthrough lines 23 and 24 to a tar header or residue line 25.

The vapor lines 21 and 17, as well as a vapor line 26 havin a pressurereducing valve 27 and leading from the still 12A, are allincommunication with a vapor header or manifold 28 which leads to adephlegrnatcr or fractionating tower 29. Thus the vapors evolved in theseveral stages of distillation which may be operating under differentpressures are taken oil? through the lines 26, 17, and 21, respectively,and by reason of the pressure reducing valves 27, 18 and 22 aredischarged at a lower pressure into the tower 29. The line 28 is shownprovided with a valve 30 for further controlling the reduction ofpressure and'delivery of vapors to the tower 29. The pressure in thetower 29 will not exceed the pressure obtaining in he lowest pressurestage distilling zone (the stills 11C and. 12C) and may, if desired, belower than that obtaining in any of the distilling stages. The tower 29is of any suitable type, such as a bubble tower or packed tower, toeffect the desired fractionation. A vapor fraction is taken off to acondenser coil 31 and the final gasoline or naphtha distillate collectedin a receiver 32. The reflux condensate is removed from the tower 29through a line 33 to a hot oil pump 34 which forces the condensatethrough a line 35 and thence into the heating coil 10A.

The reflux condensate is heated to a cracking temperature in the coil10A, preferably a higher cracking temperature than that obtaining in anyof the other cracking stages, while under a higher pressure than that ofany of the other stages. The cracking or decomposition is initiated inthe coil 10-A or carried on to the extent desired and the oil is thendischarged into the still 1l-A from which the oil undergoing crackingpasses to the still 12--A. The evolved vapors pass out through the line26 and pressure reducing valve 27 to the vapor header 28 by which theyare directed into the tower 29, as has heretofore been explained. Liquidoil is drawn off through a transfer line 36, having a pressure reducingvalve 37, and passed to the coil 10-13 to be commingled with the freshcharging stock entering through the line 16.

The stills 11A and 12A are shown equipped with lines 38 and 39,respectively, extending to a line 40 for the purpose of withdrawingliquid from the lower end of the stills. Thus when liquid is being drawnoff from the still 12A through the line 36 and passed regularly to thecoil 10-B, it is sometimes desirable to take shots or draw comparativelysmall quantities of liquid '10-." through either or both of the lines 38or 39 in order to remove from the stills heavier constituents includingcoke that may accumulate in the lower portion of these stills. In analternative m thod of operation no liquid is withdrawn 111 through theline 36 and all of the liquid is removed from the stills through eitheror both of the lines 38 and 39. The material thus directed into the line40 may be passed into the still 11-B through the line 41 or it may bepassed into the still 1lC through the line 42. The stills 11B and 12Bare shown provided with draw-off lines 43 and. 44 to either withdrawcomparatively small quantities of liquid, while regularly passing thebulk of the liquid through the transfer line 19 1 530 to the coil Ill-A,or to regularly withdraw all of the liquid from the stills 11--B and 12Bso as to pass it through the lines 40 and 4-2 to the still 11-0.

It is thus seen that in the practice of my invention the oil beingconverted is subjected to cracking conditions in a plurality of stagesunder varying pressure and temperature conditions, a residue obtained inone cracking and distilling stage being passed to successive stagesoperating at successively reduced pressures. The fresh charging stockmay be introduced to an intermediate stage of the system while thereflux condensate obtained is directed to a stage or stages operating athigher pressures and preferably also 1135 under higher temperatures thanthat of the other stages. In one modification of the invention thevapors that are collected in the line 28 from all of the distillingstages may be passed successively through a plurality of fractionatingtowers or g; fractionating sections of a tower in such a way as toobtain a plurality of reflux condensate fractions, in lieu of the singlefraction that may be withdrawn through the line 33 shown in the drawing,and these several fractions may be Ill-1t passed to separate crackingdistilling stages, the lower boiling point fractions being passed to thehigher pressure and temperature stage and the remaining fractions tocracking stages operating at lower pressures and temperatures.

In one modification of the invention the fresh charging stock, insteadof being passed to an intermediate stage of the system, may be directedto a higher pressure stage and thus the charging line 16 is shown with abranch line so as to pass the charging stock to the coil 10-A. When thismethod of operation is used it is generally preferable to introduce thereflux condensate being pumped through the line 35 to an intermediateportion of the coil 10A and a branch line 46 is shown for thus admittingthe condensate to the coil l0A. While this modified method may beemployed at times, such for example as when it is desired to charge acomparatively light charging stock to the system or when it is desiredto charge a highly refractory cycle stock that may be on hand, it isordinarily preferable to introduce the fresh charging stock to anintermediate stage of the system, such as the coil 10B and stills l1-Band 12-B.

In a preferred manner of practicing the invention a virgin stock, thatis, a stock which has not been subjected to a previous crackingoperation, is charged by the pump 15 to an intermediate cracking stagerepresented by the coil 10-13. While the practice of the invention doesnot preclude the employment of a cycle stock, or a mixture of cycle andvirgin stocks, as the fresh charging stock to the system, ordinarily ifmy invention be applied to all of the cracking operations of a refinerythere need be no occasion for the accumulation of any cycle stock intankage, since any cycle stock obtained as a result of distillation inany stage of the system may be directly used up in the process, as hasbeen explained.

Although the temperatures and pressures employed in the practice of myinvention may be varied within comparatively wide limits dependent uponthe number of cracking stages employed and other variable factors thefollowing figures may be given as a typical example of the inven tion ascarried on in three cracking stages with the fresh charging stock. beingdirected to an intermediate stage represented by the coil 10B and stills1lB and l2B and cycle condensate being passed to a highertemperature-pressure stage represented by the coil l0A and stills 11-Aand l2-A. In the higher pressure stage the pressure may, for example, bearound 600- 800 pounds at temperatures of 875-1000 F. The pressure inthe intermediate stage represented by the coil 10B and stills ll-B and12B may, for instance, be around 300-4e0 pounds at temperatures of aboutSOT-850 F. while the lowest pressure stage, represented by the coill0--C and stills l1-C and 12-C may, for example, be operated under100-200 pounds pressure at temperatures of 700-750 F. Under theseconditions the tower 29 may be operated under pressures approachingthose obtaining in the stills 11-0 or 12C or they may be reduced to suchan extent as may be desired.

It will thus be seen that my invention provides a process in whichresidues developed as a result of cracking and distillation are passedthrough cracking stages at successively lower pressures so that whilethe continued cracking of the residual oil is facilitated, as the oilpasses through the successive stages of the system, the separating outor removal of the gasoline constituents, as well as the removal of cyclestock adapted for further cracking treatment at higher temperatures andpressures, is facilitated by reason of the successive drops in pressure.

In one method of operation contemplated by the invention the lowerpressure distilling stage may be operated essentially as a stripping orauto-tar-distilling operation to recover gasoline and cycle condensatefrom the cracked residues derived from the cracking stages. Thus forexample the apparatus shown in the drawing may be operated with the coil19A and stills l1-A and 12A under a relatively high pressure, such forexample as 600-800 pounds and the stills llB and 12-B operated at alower pressure, such for example as 200-d00 pounds pressure and thestills 11-C and 12-0 operated at approximately atmospheric pressure. inthis m thod of operation it is ordinarily unnecessary to employ the coil10C, and the tar or residue that is withdrawn from the stills li--B and12-3 through the lines 43 and 4A and also any tar or residue that may bewithdrawn from the stills llA and 12-A through the lines 38 and 39 bepassed through the header 40 and line 42 nto the still 11-0. Thecontained heat of the hot residue from the cracking stills is utilizedto effect distillation in the stil s llC and l2-C, which distillationmay be aid" employment of a subatmospherio pressure. The distillation iscarried on to the extent desired to distill on" the gasolineconstituents and cycle stock and produce a residue of thecharacteristics desired which may for example be a liquid residuumadapted for fuel oil or the distillation may be carried. on to such anextent as to produce a liquid pitch which is withdrawn through either orboth of the lines 23 and 24 or, if desired, the 'stillation may becarried on o the extent of coking.

In another method of practicing the invention the temperatures employedmay be approximately the same in all of the several cracking stageswhile the pressures are progressively redu -ed.

Obviously many modifications and variations of the invention, ashereinbefore set forth, may be made without departing from the spiritand scope thereof, and therefore, only such limitations should beimposed as are indicated in the appended claims.

I claim:

that comprises subjecting fresh charging stock :1, if desired, by steamor the 1,2;

to cracking temperature under superatmospheric 1;

pressure to effect cracking and vaporization in an intermediate pressurezone, subjecting condensate oil to cracking temperature undersuperatmospheric pressure to effect cracking and vaporization in ahigher pressure zone, passing residue developed in the higher pressurecone to the intermediate zone for distillation therein, passing residuedeveloped in the int pressure zone comprising residual con from both thecracked fresh charging stock and condensate to a lower pressure zone andsuog'ecting the residue to vaporization therein, passing the vaporsevolved in several zones to a dephlegmating zone subjecting them tofractionation therein to form a vapor fraction and a reflux condensate,conducting said reflux condensate to said higher pressure zone andcondensing said vapor fraction.

2. A process in accordance with claim 1 wherein such conditions aremaintained in said lower pressure zone that the residue introducedthereinto for vaporization is reduced to coke.

CHARLES W. MACKAY.

